1. Field of the Invention
The invention relates to voltage limiters and, more specifically, to a sintering process for increasing the high temperature stability of varistors.
2. Summary Of The Prior Art
The prior art provides a wide variety of non-linear resistor devices, typically referred to as varistors. Varistors are formed by sintering a powder which typically includes zinc oxide in combination with other oxides (collectively referred to as the mixture) to form a varistor disc. Leads are attached to the disc permitting the varistor to be connected across a voltage source. Suitable packaging is provided to protect the varistor disc. The electrical characteristics of a varistor are primarily determined by the mixture, the processing of the mixture prior to sintering, and the sintering process.
The present state of the art is such that ZnO varistors will "run away" if the temperature exceeds approximately 180.degree. C. Many applications make it desirable to operate the varistors in excess of this temperature. It is believed that the characteristic V-I curve of a varistor is a result of the defect structure at the grain boundary of the ceramic body which is brought about by the addition of certain selective additives to the ZnO matrix.
The varistor ceramic is obtained as a result of blending these additives with the ZnO to form the mixture, pressing selected quantities of the mixture to form the discs and sintering the disc at an elevated temperature between 1000.degree. and 1400.degree. C. for a period of 1 to 20 hours in an atmosphere of oxygen. This sintering operation allows the formation of several distinct chemical phases within the ceramic body as well as segregation of the additives into well defined compounds. A chemical equilibrium is set up at the sintering temperature which minimizes the free energy of the materials comprising the mixture. On cooling, a non-equilibrium condition arises due to many reasons including the fact that chemical solubilities are temperature dependent, and diffusional forces become diminished to the point that mobilities of the atoms are unable to keep up with the equilibrium conditions associated with the lower temperature. This can result in a non-equilibrium condition becoming frozen in. When the disc is placed in service and the temperature increases, atoms associated with these non-equilibrium conditions migrate, resulting in an irreversible increase in the current which will lead to failure of the varistor.
As a result of the conditions discussed above, the expected life of a varistor dramatically decreases as the operating temperature of a varistor increases. For example, the irreversible resistive current of a typical varistor at a constant voltage stress (voltage applied across the varistor disc) may double in one hour at an operating temperature of 250.degree. C.
A review of the prior art clearly indicates that there is no presently recognized unified scientific theory which is applicable to predict accurately the performance of a varistor made using a particular mixture, or the effect on the characteristics of the varistor disc of the process used for making the disc. Small changes in the mixture or the sintering process can result in dramatic changes in the characteristics of the varistor. The inability to accurately predict the performance of a specific varistor without experimentation is believed to be due, in a large part, to the complex chemistry of the sintering process, discussed above. This being the case, it is necessary to select by experimentation the mixture and the process used for making the varistor disc having preselected characteristics.
Typically in manufacturing varistor discs, the mixture is sintered and, optionally, subjected to various heat treatments. For purposes of this patent application the sintering cycle and subsequent heat treatments are referred to as the "Disc Formation Process".
In designing varistors for particular applications an iterative procedure is used to select the mixture and disc formation process to produce varistors having characteristics suitable for a specific range of applications. This procedure consists of selecting a mixture based on prior experience, selecting a disc formation process based on prior experience, verifying and experimentally adjusting both the mixture and disc formation process to produce a varistor having the desired characteristics. This is the iterative procedure used in developing the disclosed invention.